1. Field of the Invention
This invention relates to a Doppler velocimeter, and more particularly to a Doppler velocimeter in which, for example, a laser beam is radiated upon an object or fluid which is moving (including fine particles in the fluid, hereinafter referred to as the "moving object" or "moving fluid") and the shift of the frequency of scattered light subjected to a Doppler shift is detected in conformity with the velocity of the moving object to thereby measure the velocity of the moving object in a non-contact manner.
2. Related Background Art
A laser Doppler velocimeter has heretofore been used as an apparatus for measuring the velocity of a moving object in a non-contact manner. The laser Doppler velocimeter is an apparatus for measuring the velocity of a moving object by the utilization of the effect that a laser beam is applied to the moving object and the frequency of scattered light from the moving object shifts in proportion to the velocity of the moving object (the Doppler effect).
FIG. 6 of the accompanying drawings is an illustration showing an example of the laser Doppler velocimeter according to the prior art for detecting chiefly the velocity of moving fluid in a tube.
In FIG. 6, a laser beam emitted from a laser (He-Ne laser) 61 is divided into transmitted light La and reflected light Lb by a beam splitter (half mirror) 64a.
The transmitted light La is converged by a converging lens 65 and enters moving fluid 68 at an angle .theta., the moving fluid 68 moving in a tube 69 at a velocity V. On the other hand, the reflected light Lb is reflected by a mirror 64b, and thereafter is converged by the converging lens 65 and enters the moving fluid 68 at an angle .theta. so as to intersect the transmitted light La in the tube 69. The transmitted light La and the reflected light Lb are set so as to substantially converge at predetermined locations in the tube 69.
If at this time, fine particles (minute particles such as impurities) P are included in the moving fluid 68, the incident light beams La and Lb will be scattered by the fine particles P. The scattered lights from the fine particles P are condensed by the converging lens 65 and a condensing lens 66 and detected by a photodetector 67, whereby they are converted into electrical signals.
At this time, the frequencies of the scattered lights by the two light beams La and Lb are subjected to Doppler shifts +.DELTA.f and -.DELTA.f, respectively, in proportion to the velocity V. When, the wavelength of the laser beam is .lambda., the variation .DELTA.f in the frequency can be represented by the following equation (1): EQU .DELTA.f=V.multidot.sin(.theta.)/.lambda. (1)
The scattered lights subjected to the Doppler shifts +.DELTA.f and -.DELTA.f interfere with each other and bring about a light-and-shade change on the light receiving surface of the photodetector 67. The frequency F thereof is given by the following equation (2): EQU F=2.multidot..DELTA.f=2.multidot.V.multidot.sin(.theta.)/.lambda.(2)
If the frequency F of the photodetector 67 (hereinafter referred to as the "Doppler frequency") is measured, the velocity V of the fine particles P, i.e., the moving fluid 68 including the fine particles p, can be found from equation (2).
In the laser Doppler velocimeter according to the prior art, as is apparent from equation (2), the Doppler frequency F is in inverse proportion to the wavelength .lambda. of the laser, and accordingly, it has been necessary to use a laser source capable of emitting a light having stable wavelength in the laser Doppler velocimeter. A gas laser such as He-Ne laser is often used as a laser source of stable wavelength capable of continuous oscillation, but such laser has required a bulky laser oscillator and a high voltage power source, and this has led to the tendency that the apparatus becomes bulky and expensive.
Also, a laser diode (or a semiconductor laser) used in a compact disk, a video disk, optical fiber communications or the like is super-compact and easy to drive, but has suffered from the problem of temperature dependence.
FIG. 7 of the accompanying drawings (reproduced from Mitsubishi Semiconductor Data Book, 1987, (Chapter: Optical Semiconductor Elements) illustrates an example of the standard temperature dependence of a laser diode, and in this figure, the portion in which the wavelength varies continuously is due chiefly to a temperature variation in the refractive index of the active layer of the laser diode, and is 0.05 to 0.06 nm/.degree.C. On the other hand, the portion in which the wavelength varies discontinuously is called longitudinal mode hopping and is 0.2 to 0.3 nm/.degree.C.
A method of controlling the laser diode to a predetermined temperature is generally adopted to stabilize the wavelength. In this method, it is necessary to mount temperature control members such as a heater, a radiator and a temperature sensor on the laser diode with small heat resistance and effect temperature control precisely. As a result, the laser Doppler velocimeter becomes relatively bulky and costly and moreover, the instability caused by the aforementioned longitudinal mode hopping cannot completely be eliminated.
Also, when the velocity (flow velocity) of fluid as a moving object or a minute particle such as a liquid droplet is to be detected, the detection is effected by detecting scattered light scattered by the minute particle or a fine particle included in the fluid and therefore, it is necessary to enclose liquid in a tube and converge a light beam in the tube or the fine particle to achieve the effective utilization of the laser beam applied. Therefore, it is necessary to use a relatively large converging lens in which aberrations are well corrected, and this has led to the problem that the entire apparatus becomes bulky and complicated.